A simple sol-gel method at low temperature was used to prepare zinc oxide (ZnO) nanoparticles. The ZnO nanoparticles which were prepared via different solvents such as water, methanol and ethanol were characterized with different techniques such as X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), ultraviolet-visible (UV-vis) spectrometry and field emission scanning electron microscopy (FESEM). The XRD results revealed that ZnO has a hexagonal structure with average crystallize size of 23.6 nm, 20.9 nm and 14.2 nm for water, methanol and ethanol based solvent respectively. The photocatalytic activities of ZnO samples were evaluated based on the removal of Congo red (CR) in aqueous solution under solar radiation. Effect of reaction time, pH, and loading of ZnO on degradation of CR was investigated, and the degradation kinetic of CR was also studied. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.